This invention relates to oral contraceptive regimens containing a non-uterotrophic antiestrogen (i.e., a tissue selective estrogen) and a progestin.
The vast majority of oral contraceptives consist of a combination of a progestin and estrogen that are administered concurrently for 21 days followed either by a 7 day pill free interval or by the administration of a placebo for 7 days in each 28 day cycle. The most important aspects of a successful oral contraceptive product are effective contraception, good cycle control (absence of spotting and breakthrough bleeding and occurrence of withdrawal bleeding), and minimal side effects. Combination oral contraceptives have traditionally acted by suppression of gonadotropins. In addition, it appears that the progestin component is primarily responsible for contraceptive efficacy through inhibition of ovulation, and other peripheral effects which include changes in the cervical mucus (which increase the difficulty of sperm entry into the uterus) and the endometrium (which reduce the likelihood of implantation). The estrogenic component intensifies the anovulatory effect of the progestin, and is also important for maintaining cycle control.
Several examples of progestin only contraceptives are known. For example products containing norethindrone (350 .mu.g) or levonorgestrel (75 .mu.g) are available, but raise several issues which limit their ultimate acceptability. The first is that currently available oral progestin only contraceptives are administered at doses that fail to completely inhibit ovulation thus pregnancy rates are marginally higher than currently available combined oral contraceptive preparations. Nonetheless, the pregnancy rates (generally less than 3 per 100 women per year) are excellent and are based primarily on cervical mucous changes and modest changes in endometrium. The second difficulty with these preparations is the extraordinarily high rate of abnormal or unexpected vaginal bleeding in women who utilize them. The absence of predictable vaginal bleeding which results from irregular development and shedding of the uterine lining (endometrium) is a phenomenon that is common to injectable, implantable and oral progestin only contraceptives. This side-effect is reported by up to 80% of women using any of these forms of progestin only contraception.
GB Patent Specification 1326528 discloses estrogen antogonizing agents (preferrably cis-clomiphene) in combination with a progestin for use as a contraceptive. The estrogen antagonists disclosed in GB 1326528 are uterotrophic (see Kumar, A. India. J. Biosc. 20(5): 665 (1995)) whereas the anti-estrogens of this invention are not.